The ligand selectivity of the three families of receptors expressed in the vomeronasal organ-V1Rs, V2Rs and FPRs-is largely unknown. To deepen our understanding of the interactions between the vomeronasal receptors and their ligands, we propose to establish systems to identify active ligands for the V2Rs and the FPRs. In addition, we propose to identify which domains of the receptor proteins determine ligand selectivity. To test the hypothesis that specific V2R or FPR vomeronasal receptors are activated by specific set of ligands, three Specific Aims are proposed: Specific Aim 1. Characterize the ligand selectivity of V2Rs. We will establish a heterologous expression system for the V2Rs. We will use this system to identify active ligands of the V2Rs and examine the structural basis of ligand selectivity by V2R family members. Specific Aim 2: Characterize the ligand selectivity of VNO-FPRs. We will establish a heterologous expression system for the VNO-FPRs. We will use this system to identify the active ligands of VNO-FPRs. Specific Aim 3: Characterize the ligand selectivity of VNO receptors using VNO sensory neurons. We will establish a system for measuring the response of dissociated vomeronasal neurons using calcium imaging and then measuring the mRNA expression in the same cells. We will use this method to identify the receptors expressed in the cells activated by a given ligand. The proposed experiments further our long term goal, which is to understand the mechanistic basis for ligand recognition by the vomeronasal receptors. The health relatedness of these studies is that a better understanding of the ligand binding attributes of vomeronasal receptors will offer insights into the general features of ligand recognition by G-protein coupled receptors, a class of receptor that is frequently targeted by drugs. This work will improve our understanding of how G protein-coupled vomeronasal receptors recognize and discriminate specific ligands. This basic knowledge will translate to greater understanding of G protein-coupled receptors, which are major pharmaceutical targets for cardiac, psychiatric, and cancer disease states. An understanding of the interactions between the ligand and receptor provides crucial information needed to develop a wide range of health-related products.